Fluorescent lamps include filaments or electrodes at each end of a glass tube, an ionizable gas, and a phosphor coating on the inside of the glass tube. When current is supplied to the filaments a voltage is induced ionizing the gas and forming an electric arc between the filaments. The electric arc generates a flow of electric current through the ionized gas causing electrons to be excited and producing light emissions. Typically, the filaments are coated with an emission mix to facilitate electron emission. The use of a ballast in a fluorescent lamp system extends the life of the lamps by preheating the filaments to mitigate the depletion of the emission mix coating.
A lamp reaches an end-of-life stage when the emission mix becomes depleted on a filament causing the lamp to draw more voltage to continue normal operation. This higher voltage results in an increase in lamp temperature which may damage the lamp or the lamp socket. A lamp at the end-of-life stage, can be replaced or, as it is sometimes technically called, relamped.
During relamping, the system can detect that the new lamp has been inserted. Conventionally, such detection has required that each lamp have a corresponding relamping detector and timing circuit. Including a separate relamping detector and timing circuit for each lamp creates extensive circuitry. Such circuitry requires a substantial footprint on a printed circuit board, a relatively large ballast, and adds to the complexity and cost of the circuitry.
In another conventional approach, all outputs of the relamping detectors are coupled to separate terminals of a timing circuit, which determines when relamping occurs. However, this arrangement also increases the complexity, size, and cost of the timing circuit.